Progress has been made in recent years in linking selective neuron loss during various neurological disorders to the actions of excitatory amino acid (EAA) neurotransmitters. EAAs such as glutamate and aspartate are bound by a family of receptors; of these, it is the NMDA receptor which is most implicated as a mediator of neurotoxicity.
In hypoxia-ischemia, hypoglycemia, seizure and brain trauma, there is excessive extracellular accumulation of EAAs in vulnerable brain regions. Moreover, N-methyl-D-aspartate (NMDA) receptor blockade protects against these insults (Choi, 1988a). Similarly, a dominant theory regarding the pathogenesis of Huntington's Disease (HD) focuses on excessive exposure to EAAs. As evidence, there is preferential loss of NMDA-receptor bearing neurons in both symptomatic and pre-symptomatic cases (Young et al.; Albin et al.). Moreover, striatal lesions induced by EAA analogs reproduce many of the neuropathologic and neurochemical features of HD (Coyle et al.; McGeer et al.; Beal et al., 1985, 1986; Davies et al.; Roberts et al.; Boegman et al.; Kowall et al.). Some investigators have even suggested that the degeneration typical of Alzheimer's Disease (AD) might be mediated by EAAs (Greenamyre et al.).
One theme common to excitotoxic insults is that they constitute crises of energy. EAA exposure (in animal models of HD) and seizure represent pathologic increases in energy demands, while hypoxia-ischemia and hypoglycemia represent pathologic disruptions of substrate delivery. In all cases, there are declines in ATP and phosphocreatine concentrations (Auer et al.).
Glucose supplementation has been shown to protect neurons from these insults. In cases of hypoglycemia glucose supplementation mitigates the primary insult: in hypoxia-ischemia, this can be shown using cell number or function as an endpoint (Goldberg et al.; Tombaugh et al.; Schurr et al.). For seizure, increased glucose availability at the time of insult attenuates damage (Meldrum; Johansen et al.; Sapolsky et al., 1989). Finally, energy depletion shifts EAAs from being neurotransmitters to being neurotoxins (Novelli et al.).
The energetic nature of these insults may be a consequence of the mechanisms of EAA and calcium trafficking. Energy depletion enhances both calcium-dependent and independent glutamate release (Drejer et al.; Dagani et al.). Furthermore, energy failure impairs high-affinity glutamate re-uptake into neurons (Drejer et al.), which enhances glutamate neurotoxicity (Kohler, et al.; Choi, 1987). Glutamate uptake into glia is also energy-dependent (Barbour et al.), and diminution of the glial component of glutamate uptake also enhances toxicity (Vibulsreth et al.).
The calcium component of neurotoxicity is also augmented by energy failure. In all of the above-described scenarios of neuron death, one consequence of the EAA excess is mobilization of free cytosolic calcium in the post-synaptic neuron. This mobilization arises from opening of a calcium channel gated to the NMDA-receptor (whose regulation is vastly complex), opening of voltage-gated calcium channels, and mobilization of calcium from intracellular stores by sodium and inositol phosphate second messengers (Choi, 1988b; Blaustein). Critically, prevention of such a rise (for example, by removal of calcium from culture media, or microinjection of calcium chelators into neurons) is neuroprotective (Choi, 1985; Goldberg et al.; Scharfman et al.).
Excessive free cytosolic calcium, in turn, is thought to be cytotoxic by indiscriminately activating various calcium-dependent proteases, lipases and nucleases, leading to free radical formation, membrane damage, DNA fragmentation, and so on (Cheung et al.). In neurons, probably the best-studied of degenerative consequences of calcium excess is the proteolysis of various cytoskeletal proteins (Siman et al.).
Experiments performed in support of the present invention support a method to buffer neurons from EAA toxicity. A herpes virus vector is used to deliver a glucose transporter (GT) gene into neurons, in order to cause overexpression and increase glucose transport into infected neurons. The experiments further demonstrate that expression of the GT gene is effective to reduce neuronal injury even when the vector is administered after the neurological insult.